RGP for Astigmatism

Question 1

Back surface toric design benefit

Answer 1

physical compatibility is good for astigmatism

Question 2

Front surface toric design

Answer 2

• spherical back surface
• cyl front surface

1. circular design
2. truncated design

• good for lenticular design

Question 3

1. Front surface toric - circular design

Answer 3

• optical zone is centred
• base down prism
  -increased comfort
  -easier manufacture + replication
• good when lower lid below limbus
• increased physiological performance

Question 4

2. Front surface toric - Truncated design

Answer 4

• optical zone decentred superiorly
• inferiorly truncated
• rests against lower lid
• prism ballast
• superior truncation to increase CL stabilising
• reduced comfort

Question 5

Bitoric design

Answer 5

• front/back surface toric gives unacceptable residual astigmatism
• toric back surface for physical fit + toric front for astigmatism correction

Question 6

Peripheral toric design

Answer 6

• spherical back optic zone
• toric back peripheral curves
• spherical front optic zone + peripheral curves
• oral/ellipsoidal shaped optic zone

Question 7

Topic GP Advantages + Disadvantages

Answer 7

ADVANTAGES:
- stabilised CL fit
- increased CL- cornea fitting relationship
- better corneal physiology than toric SCLs

DISADVANTAGES:
- thick lens
- reduced control of edge profile
- misaligned axis

Question 8

Aim of Bitoric fitting

Answer 8

• prevent redial astigmatism
• BS toric can induce astigmatism
• front toric correction
• calculate rx along each meridian
• aim for full alignment